# std::is_aggregate

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Defined in header `<type_traits>` |  |
template< class T > struct is_aggregate; |  | (since C++17)

`std::is_aggregate` is a UnaryTypeTrait.

If `T` is an aggregate type, provides the member constant `value` equal true. For any other type, `value` is false.

If `T` is an incomplete type other than an array type or (possibly cv-qualified) void, the behavior is undefined.

If the program adds specializations for `std::is_aggregate` or `std::is_aggregate_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_aggregate_v = is_aggregate<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is an aggregate type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_is_aggregate` | `201703L` | (C++17) | `std::is_agregate`

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #include <cstddef>
    #include <new>
    #include <string_view>
    #include <type_traits>
    #include <utility>

    // Constructs a T at the uninitialized memory pointed to by p using
    // list-initialization for aggregates and non-list initialization otherwise.
    template<class T, class... Args>
    T* construct(T* p, Args&&... args)
    {
        if constexpr (std::is_aggregate_v<T>)
            return ::new (static_cast<void*>(p)) T{std::forward<Args>(args)...};
        else
            return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);
    }

    struct A { int x, y; };
    static_assert(std::is_aggregate_v<A>);

    struct B
    {
        int i;
        std::string_view str;

        B(int i, std::string_view str) : i(i), str(str) {}
    };
    static_assert(not std::is_aggregate_v<B>);

    template <typename... Ts>
    using aligned_storage_t = alignas(Ts...) std::byte[std::max({sizeof(Ts)...})];

    int main()
    {
        aligned_storage_t<A, B> storage;

        A& a = *construct(reinterpret_cast<A*>(&storage), 1, 2);
        assert(a.x == 1 and a.y == 2);

        B& b = *construct(reinterpret_cast<B*>(&storage), 3, "4");
        assert(b.i == 3 and b.str == "4");
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3823 | C++17 | The behavior is undefined if `T` is an array type but `std::remove_all_extents_t<T>` is an incomplete type. | The behavior is defined regardless of the incompleteness of `std::remove_all_extents_t<T>` as long as `T` is an array type.

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